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Breast cancer screening for women over 50 saves lives, an independent panel in Britain has concluded, confirming findings in US and other studies.

But that screening comes with a cost: the review found that for every life saved, roughly three other women were overdiagnosed, meaning they were unnecessarily treated for a cancer that would never have threatened their lives.

The expert panel was commissioned by Cancer Research UK and Britain's department of health and analysed evidence from 11 trials in Canada, Sweden, the UK and the US

In Britain, mammograms are usually offered to women aged 50 to 70 every three years as part of the state-funded breast cancer screening programme.

Scientists said the British programme saves about 1,300 women every year from dying of breast cancer, while about 4,000 women are overdiagnosed. By that term, experts mean women treated for cancers that grow too slowly to ever put their lives at risk. This is different from another screening problem: false alarms, which occur when suspicious mammograms lead to biopsies and follow-up tests to rule out cancers that were not present. The study did not look at the false alarm rate.

"It's clear that screening saves lives," said Harpal Kumar, chief executive of Cancer Research UK. "But some cancers will be treated that would never have caused any harm and unfortunately, we can't yet tell which cancers are harmful and which are not."

Each year, more than 300,000 women aged 50 to 52 are offered a mammogram through the British programme.

During the next 20 years of screening every three years, 1 per cent of them will get unnecessary treatment such as chemotherapy, surgery or radiation for a breast cancer that would never be dangerous.

The review was published online yesterday in the medical journal Lancet.

Some critics said the review was a step in the right direction. Karsten Jorgensen, a researcher at the Nordic Cochrane Centre in Copenhagen, who has previously published papers on overdiagnosis, said: "Cancer charities and public health authorities have been misleading women for the past two decades by giving too rosy a picture of the benefits.

"It's important they have at least acknowledged screening causes substantial harms," he said.

Countries should now re-evaluate their own breast cancer programmes. Jorgensen said.